This invention relates to a system for controlling a vehicle safety device, such as an air bag.
An air bag control system basically comprises an acceleration sensing circuit, a collision detection circuit, and a squib drive circuit. The acceleration sensing circuit produces an output corresponding to an abrupt deceleration occurring upon collision of a vehicle. In the collision detection circuit, the output of the acceleration sensing circuit is integrated, and this integral output is compared with a threshold level. When the integral output exceeds the threshold level, a collision detection signal is outputted from the collision detection circuit. The squib drive circuit is responsive to this collision detection signal to ignite a squib so as to expand an air bag.
Conventional collision detection circuits are classified into the digital-type and the analog-type. Such a digital collision detection system provided with a microcomputer is advantageous in that it can detect collision with high precision; however, when the microcomputer runs away, a collision detection signal may be outputted even when no collision occurs, so that the squib is erroneously ignited to expand the air bag. Although the analog collision detection system does not suffer from the problem of a run-away of the microcomputer, it can not effect a high-precision collision detection.
Under the circumstances, there has been proposed an air bag control system, as disclosed in Japanese Laid-Open (Kokai) Utility Model Application No. 5371/90, which is provided with both an analog collision detection system and a digital collision detection system. A squib drive circuit of this control system ignites a squib only when the squib drive circuit simultaneously receives collision detection signals respectively from the analog and digital collision detection systems. Therefore, the ignition of the squib based on a high-precision collision detection by a microcomputer can be effected, and besides an accidental ignition of the squib due to a run-away of the microcomputer can be prevented.
In the above control system, however, a malfunction of the analog collision detection system is not detected, and there are occasions when the control system is operated with such a malfunction being present. In this case, the accidental ignition of the squib due to the run-away of the microcomputer can not be prevented by the analog collision detection system, and also in the event of the vehicle collision, the squib will fail to be ignited.
Japanese Laid-Open Patent Application No. 241467/88 (corresponding to U.S. Pat. No. 4,950,914) discloses an air bag control system which comprises a malfunction detection circuit in addition to a piezoelectric element serving as an acceleration sensor, an analog collision detection system and a squib drive circuit. The malfunction detection circuit supplies test pulses to the piezoelectric element. The piezoelectric element, when receiving the test pulse at a pair of electrodes thereof, is strained, and outputs a voltage, corresponding to this strain, from another pair of electrodes thereof. The output of the piezoelectric element produced in response to the test pulse is integrated by an integrating circuit of the analog collision detection system, and this integral output is compared with a threshold level in a comparator circuit of the analog collision detection system, and a high-level signal is outputted from the comparator circuit in a manner similar to that of the collision detection. The high-level signal of the comparator circuit is supplied to a transistor of the squib drive circuit to turn on this transistor. As a result, the voltage at one terminal of the squib increases from the ground potential to a battery voltage. The above malfunction detection circuit, when detecting this voltage increase, judges that the acceleration sensor and the analog collision detection system are normal, and the malfunction detection circuit, when not detecting the voltage increase, judges that either the acceleration sensor or the analog collision detection system is abnormal, and turns on an alarm lamp.
In the control system of the above Japanese Laid-Open Patent Application No. 241467/88, however, the malfunction detection circuit is used only to detect a malfunction of the acceleration sensor and the analog collision detection system. Further, this control system is not provided with a digital collision detection system.